Dry-etching method and apparatus, photomasks and method for the preparation thereof, and semiconductor circuits and method for the fabrication thereof

ABSTRACT

A dry-etching method comprises the step of dry-etching a metal thin film as a chromium-containing film, wherein the method is characterized by using, as an etching gas, a mixed gas including (a) a reactive ion etching gas, which contains an oxygen-containing gas and a halogen-containing gas, and (b) a reducing gas added to the gas component (a), in the process for dry-etching the metal thin film. The dry-etching method permits the production of a photomask by forming patterns to be transferred to a wafer on a photomask blank. The photomask can in turn be used for manufacturing semiconductor circuits. The method permits the decrease of the dimensional difference due to the coexistence of coarse and dense patterns in a plane and the production of a high precision pattern-etched product.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method for dry-etching a metalthin film and more specifically to a method and an apparatus fordry-etching such a metal thin film, which is applied to a process forpreparing a photomask which is used in fabricating, for instance, asemiconductor device as well as a dry-etching method which is used in apattern-etching process for forming a fine pattern of a metal thin filmsuch as fine electrode patterns (for a flat panel display (FPD) or thelike) and color filters. In addition, the present invention alsopertains to a photomask which is provided with a pattern formed usingsuch a dry-etching method and a method for preparing the photomask aswell as a semiconductor circuit fabricated using such a photomask and amethod for fabricating the semiconductor circuit.

[0003] 2. Description of the Prior Art

[0004] As a photomask blank, there have been known, for instance, thosehaving such a structure as shown in FIG. 1, which comprises a glasssubstrate a formed from, for instance, synthetic quartz glass; alight-shielding film b consisting of a thin film of a metal such aschromium, formed on the surface of the substrate; and a resist layer cof a light-sensitive/electron-sensitive resin, which is formed on thelight-shielding film. The glass substrate a may serve as a support forpatterns and therefore, must have a variety of desired characteristicproperties such as high transmittance, high uniformity, defect-freecharacteristics, resistance to washing and excellent flatness. Inaddition, the light-shielding film b may serve as a light-shieldingmaterial for patterning and therefore, should satisfy the desiredrequirements for various properties such as etching controllability,uniformity, defect-free characteristics, resistance to washing, lowstress and high adhesion to the glass substrate. Moreover, the resistlayer c has a role as a film for forming the light-shielding film andaccordingly, should have a variety of desired characteristic propertiessuch as high-sensitivity/high resolution, resistance to etching,uniformity, defect-free characteristics and high adhesion to thelight-shielding film.

[0005] A photomask provided thereon with a fine electric circuit patternhas been prepared by wet-etching or dry-etching a chromiumlight-shielding film using a photomask blank having such a structureaccording to the electron beam patterning process or the laser beampatterning process. An example of such a mask-processing scheme is shownin FIG. 2.

[0006] In the wet-etching, there have recently been highlighted a limitin the dimension control due to the undercut and a limit in theverticality of the etched cross section, and the dry-etching techniquehas thus been widely used instead.

[0007] The dry-etching methods for preparing a photomask and thedry-etching apparatus for practicing the methods are described in, forinstance, J.P. KOKAI No. Hei 6-347996, the disclosure of which is herebyincorporated by reference. In this dry-etching technique, a chromiumfilm is etched using a gas comprising, for instance, chlorine gas towhich oxygen gas is added, as a reactive ion etching gas.

[0008] Moreover, the dry-etching method for preparing a photomask of achromium-containing film is disclosed in, for instance, Japanese PatentNo. 2,765,065, the disclosure of which is hereby incorporated byreference. This patent discloses, in Examples, that when thechromium-containing film is dry-etched by this dry-etching method whileusing a resist film of a positive electron beam resist EBR-9 (which isavailable from Toray Industries, Inc.) as a mask and a mixed gascomprising 160 SCCM of chlorine gas, 40 SCCM of oxygen gas and 160 SCCMof wet air as a dry-etching gas, there is not observed any change in theetching rate of the electron beam resist film, while the etching rate ofthe chromium-containing film increases and the selective (or etching)ratio against the resist film is improved. As a result, thechromium-containing film can sufficiently be patterned by thisdry-etching technique. In this connection, the wet air (160 SCCM) in themixed etching gas comprises about 128 SCCM of nitrogen gas and about 32SCCM of oxygen gas corresponding to the component ratio of nitrogen tooxygen in the air which is equal to 4:1.

[0009] In addition, the semiconductor circuit has recently become moreand more finer and the size of the semiconductor circuit is increasinglyreduced from 0.2 μm to 0.15 μm. For instance, in case of a semiconductorcircuit fabricated using a conventional photomask, the dimensional errorobserved for the memory circuit portion is large as compared with thatobserved for the peripheral circuit portion in the memory circuit whichcomprises the memory circuit portion and the peripheral circuit portion,while such an error is also large even in the logic circuit and thusthese errors may adversely affect the characteristic properties of theresulting circuit. For this reason, there has been desired for thedevelopment of a photomask which permits the fabrication of a circuitwhose dimensional difference between circuits within a semiconductorchip is as low as possible.

[0010] If a chromium film as a light-shielding film is subjected todry-etching using a chlorine-containing gas and if a pattern is formedon a plane at an almost uniform density, the film can be chromium-etchedat an approximately uniform rate throughout the whole surface andaccordingly, the dimensional control within a plane can be achieved tosuch an extent that the in-plane uniformity 3 σ (3×the variance of(measured line width−averaged line width)) ranges from 20 to 60 nm forthe line width ranging from 1 to 2 μm.

[0011] However, dense patterns (patterns whose area occupied by a resistis small) and coarse patterns (patterns whose area occupied by a resistis large) often coexist in the plane of a practical photomask and if thedry-etching technique is used for forming such a photomask, the etchingrate of a chromium film is high at the densely patterned portion and lowat the coarsely patterned portion. As a result, the dimensionaldifference within a plane reaches up to about 100 nm for a designed linewidth ranging from 1 to 2 μm. A photomask having such a largedimensional difference within the plane cannot be used for thefabrication of, for instance, circuits having a higher integrationdensity such as memory circuits, logic circuits and LSI circuits.

[0012] The Japanese Patent No. 2,765,065 described above does not relateto the solution of the foregoing problems, but relates to theimprovement of the selective ratio of a chromium film to a resist film.

SUMMARY OF THE INVENTION

[0013] Accordingly, an object of the present invention is generally tosolve the foregoing problems associated with the conventional techniqueor an improved dry-etching technique as a means for forming a finepattern, which permits the reduction of the dimensional difference dueto the coexistence of coarse and dense patterns within a plane, forinstance, a dry-etching technique for manufacturing achromium-containing photomask. More specifically, it is an object of thepresent invention to provide a dry-etching method and a dry-etchingapparatus, which permit the production of a high precision photomask byreducing the dimensional difference due to the coexistence of coarse anddense patterns within a plane.

[0014] Another object of the present invention is to provide a methodfor preparing a photomask using the foregoing dry-etching method and toprovide a photomask thus prepared.

[0015] A further object of the present invention is to provide a methodfor fabricating a semiconductor device using the photomask and asemiconductor circuit fabricated by the method.

[0016] The inventors of the present invention have conducted variousstudies to achieve the foregoing objects, have found that even in theproduction of a photomask in which dense patterns and coarse patternscoexist in the plane thereof, the use of a mixed etching gas comprisingan oxygen-containing halogen gas such as an oxygen-containing chlorinegas (e.g., Cl₂+O₂), to which at least a hydrogen-containing gas (e.g.,H₂, hydrogen chloride (HCl) gas) is added, in the etching of thechromium film permits the achievement of in-plane dimensional controlalmost identical to that achieved for a mask in which patterns areformed in the plane at an almost uniform density, i.e., such in-planedimensional control that the dimensional difference is not more than ahalf of that conventionally attained, for instance, 10 to 50 nm (0.010to 0.050 μm) and thus have completed the present invention on the basisof such a finding for the designed line width ranging from 1 to 2 μm.

[0017] According to a first aspect of the present invention, there isprovided a dry-etching method characterized by using, as an etching gas,a mixed gas including (a) at reactive ion etching gas, which contains anoxygen-containing gas and a halogen-containing gas, and (b) a reducinggas added thereto, in a process for dry-etching a metal thin film.

[0018] According to a second aspect of the present invention, there isprovided a method for preparing a photomask by performing a series ofpattern-forming steps such as a step for forming a resist layer on aphotomask blank, a step for exposing and patterning the resist layer, adeveloping step, a step for etching the photomask blank and a step forremoving the resist layer and which is characterized in that patterns tobe transferred onto a wafer are formed on the photomask blank accordingto the dry-etching method described above to thus give a photomask.

[0019] According to a third aspect of the present invention, there isprovided a photomask which is prepared through a series ofpattern-forming steps such as a step for forming a resist layer on aphotomask blank, a step for exposing and patterning the resist layer, adeveloping step, a step for etching the photomask blank and a step forremoving the resist layer and which is characterized in that patterns tobe transferred onto a wafer are formed on the photomask blank accordingto the dry-etching method described above to thus give a photomask.

[0020] According to a fourth aspect of the present invention, there isprovided a method for manufacturing a semiconductor circuit whichcomprises the steps of transferring the patterns formed on the photomaskaccording to the third aspect of the invention on a wafer on which alight-sensitive material is coated, developing the light-sensitivematerial to form resist patterns on the wafer, to manufacture asemiconductor circuit which comprises coexisting coarse and densepatterns corresponding to the resist patterns.

[0021] According to a fifth aspect of the present invention, there isprovided a semiconductor circuit which has a circuit comprisingcoexisting coarse and dense patterns corresponding to the resistpatterns formed by transferring the resist patterns formed on thephotomask according to the third aspect of the invention on a wafer onwhich a light-sensitive material is coated and then developing thelight-sensitive material.

[0022] According to a sixth aspect of the present invention, there isprovided a dry-etching apparatus used in dry-etching a metal thin film,which is provided with a sequencer for establishing dry-etchingconditions, wherein the metal thin film is a chromium-containing film;wherein if an etching gas used consists of chlorine, oxygen and hydrogengases, the relative flow rates of these gases as expressed in terms of %by volume range from 73 to 46, 19 to 11 and 9 to 42% by volume,respectively, or if an etching gas used consists of chlorine, oxygen andhydrogen chloride gases, the relative flow rates of these gases asexpressed in terms of % by volume range from 70 to 36, 18 to 9 and 13 to55% by volume, respectively; and wherein the apparatus is designed insuch a manner that when inputting the parameters relating to theforegoing dry-etching conditions, directly or through a memory device ofa computer, to the sequencer and then starting the dry-etching process,the dry-etching is automatically carried out under the foregoingdry-etching conditions.

[0023] According to a seventh aspect of the present invention, there isprovided a dry-etching apparatus which comprises an etching chamber, atransport chamber, a substrate cassette bed and a sequencer forestablishing dry-etching conditions, wherein four electromagnets eachcomprising a square-shaped ring-like coil are provided on the outer sideof the etching chamber, two each of these electromagnets being oppositeto one another and making a pair, these electromagnets being so designedthat when applying a low frequency current which is 90 deg. out of phasethereto, the combined magnetic field established by these two pairedelectromagnets can rotate in a plane parallel to a substrate at afrequency identical to that of the low frequency current, an RFelectrode and an opposite electrode are disposed in the etching chamber,a transport robot for transporting the substrate is provided in thetransport chamber, the transport robot being a two-joint robot havingtwo knots, the tip of a transport arm thereof being able to undergoadvancing, reciprocating and rotating motions due to the composition ofrotational motions of a motor axis and these two knots within eachhorizontal plane, the robot thus transporting the substrate, wherein ametal thin film to be dry-etched is a chromium-containing film, whereinif an etching gas used consists of chlorine, oxygen and hydrogen gases,the relative flow rates of these gases as expressed in terms of % byvolume range from 73 to 46, 19 to 11 and 9 to 42% by volume,respectively, or if an etching gas used consists of chlorine, oxygen andhydrogen chloride gases, the relative flow rates of these gases asexpressed in terms of % by volume range from 70 to 36, 18 to 9 and 13 to55% by volume, respectively, and wherein the apparatus is designed insuch a manner that when inputting the parameters relating to theforegoing dry-etching conditions, directly or through a memory device ofa computer, to the sequencer and then starting the dry-etching process,the dry-etching is automatically carried out under the foregoingdry-etching conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The aforementioned and other objects, features and advantages ofthe present invention will be become more apparent from the followingdescription taken with reference to the accompanying drawings, wherein

[0025]FIG. 1 is a cross sectional view showing the structure of aphotomask blank;

[0026]FIG. 2 is a mask process flow diagram for explaining the processfor preparing a photomask;

[0027]FIG. 3 is a partially cutaway plan view showing a dry-etchingapparatus used for carrying out the present invention;

[0028]FIG. 4 is a cross sectional view of the dry-etching apparatusshown in FIG. 3 taken along the line A-A;

[0029]FIG. 5(A) is a schematic plan view showing the arrangement of testpatterns used in Example 1 and

[0030]FIG. 5(B) shows the pattern arrangement as shown in FIG. 5(A) andis a schematic plan view showing the positions (or lines A-A′ and B-B′)along which the cross sectional views shown in FIGS. 6(A) and 6(B) aretaken;

[0031]FIG. 6(A) is a flow diagram for explaining the preparation ofmeasuring patterns (or test patterns) at a densely patterned portionalong the line (A-A′) in FIG. 5(B), and

[0032]FIG. 6(B) is a flow diagram for explaining the preparation ofmeasuring patterns at a coarsely patterned portion along the line (B-B′)in FIG. 5(B);

[0033]FIG. 7 is a graph showing the influence of the flow rate of addedhydrogen gas to the etching gas, on the change of the etching rate ofchromium and resist and that of range/average;

[0034]FIG. 8 is a graph showing the influence of the addition ofhydrogen gas to the etching gas, on the change of the dimensionaldifference due to the coexistence of coarse and dense patterns observedwhen the test pattern shown in FIG. 5 is formed by etching;

[0035]FIG. 9 is a graph showing the influence of hydrogen gas to theetching gas, on the change of the dimension of densely patterned portionobserved when the test pattern shown in FIG. 5 is formed by etching;

[0036]FIG. 10 is a graph showing the influence of the flow rate of addedhydrogen gas to the etching gas, on the change of the dimension ofcoarsely patterned portion observed when the test pattern shown in FIG.5 is formed by etching;

[0037]FIG. 11 is a graph showing the influence of the addition ofhydrogen chloride gas or NH₃ gas to the etching gas, on the change ofthe dimensional difference due to the coexistence of coarse and densepatterns observed when the test pattern shown in FIG. 5 is formed byetching, in comparison with the influence of hydrogen gas;

[0038]FIG. 12 is a graph showing the influence of the flow rate of addedhydrogen chloride gas to the etching gas, on the change of the etchingrate of chromium and resist and that of range/average;

[0039]FIG. 13 is a graph showing the influence of the flow rate of addedNH₃ to the etching gas, on the change of the etching rate of chromiumand resist and that of range;

[0040]FIG. 14 is a schematic diagram showing the structure of a memorycircuit of the present invention, which comprises a memory circuitportion and a peripheral circuit portion; and

[0041]FIG. 15 is a schematic diagram showing the structure of a systemLSI circuit of the present invention which comprises combined memorycircuit and logic circuit portions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] The dry-etching method according to the present invention iscarried out using, as a reactive etching gas, a mixed gas comprising (a)a reactive etching gas, which consists of an oxygen-containing gas and ahalogen-containing gas, and (b) a reducing gas added thereto in theprocess for dry-etching a metal thin film. This metal thin metalincludes a thin film comprising Al, Au, Pt, Ag, Si, Ge, Cr, Fe, Cu, Ni,Ta, Mo, W or Zr, or an alloy of two or more metals selected from thesemetals. In addition, this metal thin film may be a thin film such as ametal film, a metal oxide film, a metal nitride film, a metal fluoridefilm or a laminated film thereof.

[0043] In addition, the reducing gas used herein may be a gas containingat least hydrogen, i.e., hydrogen gas; a hydrocarbon gas selected fromthe group consisting of C_(n)H_(2n+2) (n=1 to 8), C_(n)H_(2n) (n=2 to10), C_(n)H_(2n−2) (n=2 to 8); an alcoholic gas selected from the groupconsisting of CH₃OH, C₂H₅OH, CH₃CH₂CH₂OH, (CH₃)₂CHOH, (CH₃)₃COH,CH₂═CHCH₂OH; a hydrogen halide gas selected from the group consisting ofHF, HCl, HBr and HI; ammonia gas; or water.

[0044] If the metal thin film is a chromium-containing film, and if theetching gas or the mixed gas used consists of chlorine, oxygen andhydrogen gases, the flow rates of these gases as expressed in terms of %by volume preferably range from 73 to 46, 19 to 11 and 9 to 42% byvolume, respectively, while if the mixed gas used consists of chlorine,oxygen and hydrogen chloride gases, the flow rates of these gases asexpressed in terms of % by volume preferably range from 70 to 36, 18 to9 and 13 to 55% by volume, respectively. This is because if the flowrates each is beyond the foregoing range, it is difficult to obtain ahighly precision photomask having a reduced dimensional difference dueto the coexistence of coarse and dense patterns in a plane.

[0045] Moreover, the oxygen-containing gas used in the invention may bea gaseous oxygen-containing compound which can be an oxygen source, suchas O₂, CO, CO₂, NO and N₂O and the halogen-containing gas usable hereinmay be a gaseous halogen (such as chlorine, fluorine)-containingcompound such as chlorine gas, CCl₄, CF₂Cl₂, CFCl₃ or CF₃Cl, which is acommonly used reactive ion etching gas.

[0046] The dry-etching apparatus used for practicing the method ofdry-etching the metal thin film according to the present invention isnot restricted to any particular one and may be, for instance, anapparatus which makes use of a variety of systems such as barrel type,RIE, MERIE, ICP, NLD and ECR. Preferred are those depicted in FIGS. 3and 4, which are equipped with a sequencer for establishing dry-etchingconditions, wherein if the metal thin film is a chromium-containing filmand if the mixed gas used as etching gas consists of chlorine, oxygenand hydrogen gases, the flow rates of these gases as expressed in termsof % by volume range from 73 to 46, 19 to 11 and 9 to 42% by volume,respectively; or wherein if the mixed gas used as etching gas consistsof chlorine, oxygen and hydrogen chloride gases, the flow rates of thesegases as expressed in terms of % by volume range from 70 to 36, 18 to 9and 13 to 55% by volume, respectively; and wherein the apparatus isdesigned in such a manner that when inputting the parameters relating tothe foregoing dry-etching conditions, directly or through a memorydevice of a computer, to the sequencer and then starting the dry-etchingprocess, the dry-etching is automatically carried out under theforegoing dry-etching conditions.

[0047] The dry-etching apparatus equipped with the foregoing sequenceraccording to the present invention comprises an etching chamber, atransport chamber and a substrate cassette bed, wherein fourelectromagnets each comprising a square-shaped ring-like coil areprovided on the outer side of the etching chamber, two each of theseelectromagnets being opposite to one another and making a pair, theseelectromagnets being so designed that when applying a low frequencycurrent which is 90 deg. out of phase thereto, the combined magneticfield established by these two paired electromagnets can rotate in aplane parallel to the substrate at a frequency identical to that of thelow frequency current, an RF electrode and an opposite electrode beingdisposed in the etching chamber, a transport robot for transporting thesubstrate being provided in the transport chamber, the transport robotbeing a two-joint robot having two knots, the tip of the transport armthereof being able to undergo advancing, reciprocating and rotatingmotions due to the composition of rotational motions of a motor axis andthese two knots within each horizontal plane, and the robot thustransporting the substrate.

[0048] In Examples given later, all of the pressure, RF electric power,magnetic field, distance between electrodes, kinds of etching gases andthe flow rate ratio: Cl₂/O₂ in the etching gas mixture are fixed topredetermined values respectively, but they are not restricted to thesespecific values and the dry-etching operations may be performed underthe following conditions, if an MERIE apparatus is, for instance, used:Pressure: 1.3 to 66.7 Pa (10 to 500 mTorr) RF Electric Power: 10 to 300W (RF Electric Power density: 0.10 to 0.4 W/cm²) Flow Rate Ratio,O₂/(Cl₂ + O₂): 10 to 25% Cl₂/O₂: 20 to 160/5 to 100 SCCM Magnetic Field:0 to 150 Gs Interelectrode Distance: 40 to 120 mm

[0049] According to the present invention, a photomask can be preparedby a series of well-known pattern-forming steps such as a step forforming a resist layer on a photomask blank, a step for exposing andpatterning the resist layer, a developing step, a step for etching thephotomask blank and a step for removing the resist layer, whereinpatterns to be transferred onto a wafer are formed oil the photomaskblank using the dry-etching method described above as the dry-etchingprocess. In addition, the photomask of the present invention ischaracterized in that patterns to be transferred onto a wafer are formedon a photomask blank using the dry-etching method described above as thedry-etching process among a series of the foregoing well-knownpattern-forming steps.

[0050] The present invention further permits the manufacture of asemiconductor circuit by transferring the resist patterns formed on thephotomask produced by the foregoing method on a wafer on which alight-sensitive material is coated, developing the light-sensitivematerial to form the resist patterns on the wafer, and then subjectingthe wafer to etching such as dry-etching or ion-implantation on thebasis of the resist patterns thus formed on the wafer to thereby form acircuit having patterns corresponding to the resist patterns. Examplesof semiconductor circuits thus obtained include a memory circuit inwhich patterns are regularly arranged, a logic circuit comprisingrandomly arranged patterns, and a system LSI circuit comprising combinedmemory and logic circuits.

[0051] The following are characteristic properties of the semiconductorcircuit obtained using the photomask of the present invention. Forinstance, a memory circuit comprises a memory circuit portion on whichpatterns are regularly arranged and a peripheral circuit portion onwhich patterns are irregularly arranged to ensure the connection to theexterior and therefore, the areas occupied by the patterns in thesecircuit portions are different from one another. More specifically, in agate-forming process in manufacturing a transistor which has animportant influence upon the characteristics of the resulting circuit,the rate of area to be removed for patterning in the peripheral circuitportion is high as compared with that observed for the memory cellportion. The semiconductor circuit has recently become more and morefiner and the size of the semiconductor circuit is increasingly reducedfrom 0.2 μm to 0.15 μm. In case of the semiconductor circuit fabricatedusing the photomask according to the present invention, the dimensionaldifference observed between the memory cell and peripheral circuitportions is very small, the variation in the dimension is also small andtherefore, the characteristics of the circuit are not adversely affectedat all. For this reason, the present invention permits the manufactureof an excellent semiconductor circuit whose memory cell and peripheralcircuit portions have almost the same characteristic properties.

[0052] The same effect is also observed for the logic circuit in whichpatterns are randomly arranged and the distribution of the area to beremoved for patterning is also random and thus the invention permits themanufacture of a quite excellent semiconductor circuit having a very lowdimensional difference within the chip. The present invention canfurther be applied to the production of a system LSI circuit comprisinga combination of a memory circuit portion and a logic circuit portion.In this case, the memory circuit portion and the logic circuit portiondiffer from each other in the packing densities of devices and thedensities of wiring, but the dimensional difference between the memoryand logic circuit portions is very small and therefore, does notadversely affect the characteristic properties of the resulting circuit.As a result, a good semiconductor circuit can thus be produced, whichdoes not have any difference between the memory and logic circuitportions in their characteristics.

[0053] The present invention will hereinafter be described in moredetail with reference to the following Examples and attached figures,but these Examples are given only for the purpose of illustration andthe present invention is not restricted to these specific Examples atall.

[0054] The dry-etching apparatus (MERIE apparatus) used in the followingExamples is shown in FIGS. 3 and 4. This dry-etching apparatus 1 is sodesigned that an etching chamber 2, a transport chamber 3 and asubstrate cassette bed 4 are accommodated in a panel 12 whichconstitutes the outer periphery of the dry-etching apparatus and thatthe etching chamber 2 is accommodated in electromagnets 5, 6, 7, 8disposed on the outer periphery of the chamber 2. Each electromagnetcomprises a square-shaped ring-like coil, the electromagnets 5 and 6 andthe electromagnets 7 and 8 make pairs respectively and a low frequencycurrent whose phase is shifted, for instance, 90 deg. is passed throughthese electromagnets. These electromagnets are so designed that thecoils of the paired electromagnets are wound in the same direction andthe combined magnetic field established by these two pairedelectromagnets 5 and 6, and 7 and 8 can rotate in a plane parallel tothe substrate at a frequency identical to that of the low frequencycurrent as shown by the dotted arrows in FIGS. 3 and 4. Disposed withinthe etching chamber 2 are a plate-like RF electrode 10 which isconnected to an RF power supply 9 through a condenser 13 and aplate-like opposite electrode 14 and a substrate 11 may be placed on theRF electrode 10 through a substrate delivery port 15 formed on the sideof the etching chamber 2.

[0055] The opposite electrode 14 and the etching chamber 2 aremaintained at the ground voltage. To supply a reactive gas for etchingto the etching chamber 2, a gas supply system 16 is disposed at areactive gas supply port 30, which is provided with a gas bomb and amass flow controller and an exhaust system 17 is connected to a vacuumexhaust port 18 of the etching chamber 2, which is equipped with avacuum pump for controlling the gas pressure in the etching chamber 2.The reactive gas herein used is one comprising an oxygen-containingmixed gas and a reducing gas at least containing hydrogen, as hasalready been discussed above.

[0056] A plurality of substrates 11 are accommodated in a cassette case19, then the cassette case is put on the substrate cassette bed 4, eachsubstrate delivered from the cassette case 19 is brought into thetransport chamber 3 through a partition valve 20 by the action of atransport robot 21 and then placed on the RF electrode 10 in the etchingchamber 2 through a vacuum valve 22 and the substrate delivery port 15.The transport robot 21 is a known two-joint robot having two knots 26and 28 and is so designed that the tip of a transport arm 29 thereof canundergo reciprocating and rotating motions due to the composition ofrotational motions of a motor axis 24 and these two knots 26 and 28. Themotions of a first arm 25 and a second arm 27 are restricted to onlythose within the horizontal planes. The movement of the tip of thetransport arm 29 between the cassette case 19 and the transport chamber3 through the partition valve 20 and that of the tip of the transportarm 29 between the RF electrode 10 in the etching chamber 2 and thetransport chamber 3 through the substrate delivery port 15 are advancingand reciprocating motions due to the composition of rotational motionsof each arm 25, 27, 29 of the robot at the motor axis 24 and theforegoing two knots 26 and 28. The transport of the substrate betweenthe vacuum valve 22 at the upstream side of the substrate delivery port15 and the partition valve 20 on the side of the cassette case 19 isperformed by the half turn motion, within a horizontal plane, of thetransport arm 29 of the robot, wherein the motor axis 24 serves as arotating center. If a motor 23 disposed on the exterior of the transportchamber 3 is rotated, the plate-like transport arm 29 carrying asubstrate 11 undergoes reciprocating and rotational motions to thustransport the substrate between the cassette case 19 and the RFelectrode 10.

[0057] When etching a pattern-forming material of the substrate 11 onthe RF electrode 10, the etching chamber 2 is evacuated by operating theexhaust system 17, followed by introduction of a reactive gas into thechamber 2 through the reactive gas supply port 30, excitation of the twopairs of electromagnets 5, 6, 7, 8 and application of an RF electricpower to the RF electrode 10 to thus generate plasma. If the same lowfrequency alternating current is passed through these two pairedelectromagnets 5 and 6, and 7 and 8 in the same direction, whileshifting the phase of the current applied to either of the pairedelectromagnets 90 deg. relative to that of the other current, a rotatingmagnetic field is established in a plane parallel to the substrate 11.The plasma generated between the RF electrode 10 and the oppositeelectrode 14 is concentrated on the surface of the substrate 11 by theaction of the rotating magnetic field and this leads to an increase ofits density. Thus, the reactive gas introduced into the chamber ishighly efficiently dissociated and the substrate 11 is subjected toreactive ion-etching under such a condition that only a slight DC biasvoltage is generated on the substrate.

[0058] For instance, it is assumed that the dry-etching is carried outusing, as the substrate 11, a photomask substrate which comprises atransparent substrate of synthetic quartz or the like, a thin layer of apattern-forming material such as Cr, Cr provided with an antireflectionfilm or SiO₂ which is applied onto the transparent substrate and apatterned photoresist layer (for instance, a resist for EB exposure(e.g. ZEP-810S (trade name) available from Nippon Zeon Co., Ltd.))provided on the pattern-forming material. In this case, if the substrate11 is dry-etched with the introduced reactive gas under the sameconditions used for the dry-etching of the IC substrate, this results ininsufficient selection ratio of the material to be etched to the resistand insufficient in-plane dimensional uniformity of the photomask sincethe photomask substrate 11 is not made of silicon, but is made fromsynthetic quartz unlike the IC substrate and the pattern-formingmaterial applied thereon does not comprise poly Si or an oxidelayer+poly Si, but comprises Cr, Cr provided with an antireflection filmor SiO₂, unlike the IC substrate. However, the selection ratio and thein-plane dimensional uniformity can be improved and a highly precisionphotomask can be produced if establishing the following dry-etchingconditions: a magnetic field intensity ranging from 50 to 150 Gs; apressure of the reactive gas in the etching chamber 2 ranging from 0.03to 0.3 Torr (4 to 40 Pa); and an RF electric power density on the RFelectrode 10 ranging from 0.20 to 0.32 W/cm².

[0059] The following are specific Examples of the present invention.

EXAMPLE 1

[0060] A dry-etching method carried out according to the presentinvention will be described in this Example. Test patterns used in thisExample are shown in FIGS. 5(A) and 5(B) and the flow diagrams forillustrating the preparation of test samples are shown in FIGS. 6(A) and6(B). FIG. 6(A) is a flow diagram, as expressed in terms ofschematically cross sectional diagrams, for explaining the preparationof measuring patterns at a densely patterned portion along the line(A-A′) in FIG. 5(B), and FIG. 6(B) is a flow diagram, as expressed interms of schematically cross sectional diagrams, for explaining thepreparation of measuring patterns at a coarsely patterned portion alongthe line (B-B′) in FIG. 5(B). As will be clear from the flow diagramsshown in FIGS. 6(A) and (B), each test sample was prepared throughprocesses for (a) EB patterning, (b) developing, (c) etching and (d)removing the resist. In FIGS. 6(A) and (B), d represents a substrate, erepresents a chromium film and f represents a resist layer. The detailsof the etching processes shown in FIGS. 6(A) and (B) are as follows:

[0061] The conditions for operating the MERIE apparatus as shown inFIGS. 3 and 4, which permit the dry-etching of a chromium film using aconventional chlorine-containing gas have already been described above,but in this Example, the test patterns as shown in FIGS. 5(A) and (B)were formed by dry-etching a chromium photomask blank under theconditions disclosed in Table 1, using an etching gas comprising theforegoing gas system (Cl₂/O₂=80/20 SCCM) to which hydrogen gas was addedin an amount specified in the following Table 1 and using the MERIEapparatus as shown in FIGS. 3 and 4, on the basis of the followingworking conditions of the apparatus among others: Pressure: 6.8 Pa (50mTorr) RF Power Supply: 80 W Gas: Cl₂/O₂ = 80/20 SCCM Magnetic Field: 50to 60 Gs Interelectrode Distance: 60 mm

[0062] The photomask blank used in this Example was one prepared byapplying a single chromium layer in a thickness of about 800 to 900 Å asa pattern-forming material onto the surface of a square-shaped syntheticquartz substrate having a size of 152.4×152.4 mm and a thickness of 6.35mm or further applying a single or two chromium layer provided with achromium oxide-containing antireflection film in a thickness of about150 Å thereon and then applying a resist layer (ZEP-810S (trade name)available from Nippon Zeon Co., Ltd.) for EB exposure onto the layer ofthe pattern-forming material. Moreover, the test patterns as shown inFIGS. 5(A) and (B) comprised, after the EB exposure and the development,a dimension-evaluation pattern (about 6.5 mm×35 mm) which was arrangedat the central portion of the left half (dense portion) of the mask andwhich included a plurality of L/S (Line and Space), ISO Line (IsolatedL) and ISO Space (Isolated S) patterns therein; a completely removedpattern (the pattern of the exposed chromium having a size of 46 mm×54mm) surrounding these patterns; and a dimension-evaluation pattern whichwas arranged at the central portion of the right half (coarse portion)of the mask.

[0063] Shown in FIG. 7 are the results thus observed when adding 0 to72.1 SCCM of hydrogen gas to the etching gas, i.e. variations in theetching rate of chromium and resist and in the range/average as afunction of the added amount of the hydrogen gas. In FIG. 7, the resultswere obtained by using a chromium mask blank and a chromium mask blankon which the above-mentioned EB resist is applied but is not patterned.As for the right longitudinal axis in FIG. 7, the range represents thedifference between the maximum value and minimum value of a thickness ofthe resist film in a plane which were determined after etching and theaverage represents a mean value of the in-plane resist film thickness,the range/average being expressed in terms of %.

[0064] Shown in FIG. 8 are the results observed when forming testpatterns (FIGS. 5(A) and (B)) which comprised dense and coarse patternsarranged in a plane, or the variation of the dimensional differencebetween the coarse and dense portions (i.e., the difference in thedimension between the coarse and dense portions) as a function of theadded amount of the hydrogen gas. In addition, shown in the followingTable 2 and FIGS. 9 and 10 are the changes in the dimension of thecoarse and dense portions as a function of the amount of the hydrogengas which is added to the etching gas when the test patterns are formedthrough dry-etching. Moreover, the data shown in FIG. 8 are shown inFIG. 11 as a function of the added amount of the gases while comparingthese results with those obtained in the other Examples. Shown in thefollowing Table 3 are the dimensional variation in the coarse and denseportions of the resist pattern obtained after the development as afunction of the added amount of the hydrogen gas, hydrogen chloride gasor NH₃ gas.

[0065] The following were found, as a result of the dry-etchingperformed under the conditions as mentioned in Table 1:

[0066] As will be seen from the results shown in FIG. 7, it was observedthat when the hydrogen gas is added to the etching gas, the etching rateof chromium increases as the added amount of the hydrogen gas increases,is at the highest when adding about 20 to 45 SCCM of the hydrogen gasand decreases as the added amount of the hydrogen gas increases evenfurther; the etching rate of the resist increases as the added amount ofthe hydrogen gas increases; and as for the in-plane resist filmthickness, the range/average, i.e. ((Maximum film thickness-Minimum filmthickness)/mean film thickness)×100 (%), decreases as the added amountof the hydrogen gas increases.

[0067] As will be seen from the results shown in Table 2 and FIGS. 8 and11, there was observed a dimensional difference of about 90 nm (0.090μm) between the coarse and dense portions when any hydrogen gas was notadded to the etching gas, while the dimensional difference therebetweenis reduced as the added amount of the hydrogen gas increases. Morespecifically, when an increasing amounts of hydrogen gas ranging from 10to 72.1 SCCM were added as in Sample Nos. 1-2, 1-3, 1-4 and 1-5, thedimensional difference between the coarse and dense portions of thepattern was found to be about 0.040 to 0.009 μm which corresponded toabout {fraction (1/2.2)} to {fraction (1/10)} times that (0.090 μm)observed when any hydrogen gas was not added. This clearly indicatesthat the problem of the dimensional difference between the coarse anddense portions is considerably eliminated. As shown in Table 1, the gasflow rates of Cl₂, O₂ and H₂ and the relative flow rates (as expressedin terms of % by volume) used in the dry-etching of the samples 1-2,1-3, 1-4 and 1-5 are 80,20 and 10 to 72.1 SCCM and 72.73 to 46.48, 18.18to 11.62 and 9.09 to 41.89% by volume, respectively.

[0068] As will be seen from the results shown in Table 1 and FIGS. 9 and10, it was observed that the dimensions of the dense portions do notalmost change due to the addition of the hydrogen gas, while thedimension of the coarse portion changes by a large amount until theadded amount of the hydrogen gas reaches 30 SCCM and the change in thedimension of the coarse portion is reduced when the added amount ofhydrogen gas is beyond 30 SCCM. As a result, the dimension of the coarseportion is close to that of the dense portion and therefore thedimensional difference between the dense and coarse portions becomessmall.

[0069] As has been discussed above, it has been proved that theconditions for dry-etching the samples 1-2, 1-3, 1-4 and 1-5 are optimumones and accordingly, the apparatus was so designed that the parametersrelating to the foregoing dry-etching conditions listed in Table 1 wereinputted, directly or through a memory device of a computer, to thesequencer and then the dry-etching process was started to automaticallycarry out the dry-etching under the foregoing optimum dry-etchingconditions.

EXAMPLE 2

[0070] A dry-etching method carried out according to the presentinvention will be described in this Example. The low reflection chromiumphotomask blank and the test patterns used in this Example were the sameas those used in Example 1. The scheme for preparing the test sampleswere the same as that used in Example 1 except that ZEP7000 (the tradename of a product available from Nippon Zeon Co., Ltd.) was substitutedfor the ZEP-810S as the resist for the EB exposure in the EB patterningprocess and that hydrogen chloride (HCl) gas, as the gas to be added tothe etching gas, was substituted for the hydrogen gas (H₂) used inExample 1 in the etching step. The pattern-forming conditions are listedin the following Table 1 together with those used in Example 1.

[0071] HCl gas was added to the etching gas in an amount ranging from 0to 120 SCCM in this Example. Regarding the results observed aftercarrying out the etching under the conditions specified in Table 1, thechange in the etching rate of each of chromium and resist and that inthe range/average are shown as a function of the added amount of HCl gasin FIG. 12. In FIG. 12, the results were obtained by using a chromiummask blank on which the above-mentioned EB resist is applied, withoutany patterning, as in this case of FIG. 7. As for the right longitudinalaxis in FIG. 12, the range/average is the same as that shown in FIG. 7.

[0072] Shown in FIG. 11 are the results observed when forming testpatterns (FIGS. 5(A) and (B)) which comprised dense and coarse patternsarranged in a plane. Namely, the change in the averaged dimensionaldifference between the coarse and dense portions (i.e., the differencein the dimension between the coarse and dense portions) as a function ofthe added amount of the HCl gas was shown in FIG. 11 while comparingthese results with those obtained in Example 1 in which hydrogen gas wasused as the added gas component. In addition, shown in the followingTable 2 are the change in the dimension of the coarse and dense portionsas a function of the amount of the hydrogen chloride gas which is addedto the etching gas when the test patterns are formed throughdry-etching, together with the results obtained in Example 1 in whichhydrogen gas is added. Shown in the following Table 3 are thedimensional variation in the coarse and dense portions of the resistpattern obtained after the development as a function of the added gasand an added amount thereof.

[0073] As will be seen from the results shown in FIG. 12, when addingthe hydrogen chloride gas to the etching gas, the etching rate ofchromium is at the highest when adding about 30 to 40 SCCM of thehydrogen chloride gas and decreases as the added amount of the hydrogenchloride increases even further, while the etching rate of chromiumincreases by increasing an amount of oxygen. In addition, it wasobserved that the etching rate of the resist increased until the addedamount of the hydrogen chloride gas reaches 15 SCCM and, even if theadded amount of the hydrogen chloride gas increased even further, theetching rate was kept at a constant rate until the added amount reaches120 SCCM.

[0074] As will be seen from the data shown in Table 2 and FIG. 11, theresults of Sample Nos. 2-2, 2-3, 2-4 and 2-5 indicate that when HCl gaswas added in an amount ranging from 15 to 120 SCCM to the etching gas,the dimensional difference between the coarse and dense portions of thepatterns was in the range of from about 0.027 to 0.050 μm whichcorresponded to about {fraction (1/3.2)} to {fraction (1/1.8)} timesthat (0.088 μm) observed when any HCl gas was not added. This clearlyindicates that the problem of the dimensional difference between thecoarse and dense portions is considerably eliminated. The result ofdimensional difference may further be improved by optimizing therelative flow rates of gas components in the etching gas. As shown inTable 1, the gas flow rates of Cl₂, O₂ and HCl and the relative flowrates (expressed in terms of % by volume) used in the dry-etching of thesamples 2-2, 2-3, 2-4 and 2-5 are 80, 20 and 15 to 120 SCCM; and 69.57to 36.36, 17.39 to 9.09 and 13.04 to 54.54% by volume, respectively.

[0075] Moreover, the etching operation may be safer by the use of HClgas in place of hydrogen gas as the added gas.

[0076] As has been described above in detail, it has been proved thatthe conditions for dry-etching the samples 2-2, 2-3, 2-4 and 2-5 areoptimum ones and accordingly, the apparatus was so designed that theparameters relating to the foregoing dry-etching conditions listed inTable 1 were inputted, directly or through a memory device of acomputer, to the sequencer and then the dry-etching process was startedto automatically carry out the dry-etching under the foregoing optimumdry-etching conditions.

EXAMPLE 3

[0077] A dry-etching method carried out according to the presentinvention will be described in this Example. The low reflection chromiumphotomask blank and the test patterns used in this Example were the sameas those used in Example 1. The scheme for preparing the test sampleswere the same as that used in Example 1 except that ZEP7000 (the tradename of a product available from Nippon Zeon Co., Ltd.) was substitutedfor the ZEP-810S as the resist for the EB exposure in the EB patterningprocess and that NH₃ gas, as the gas to be added to the etching gas, wassubstituted for the hydrogen gas (H₂) used in Example 1 in the etchingstep. The pattern-forming conditions are listed in the following Table 1together with those used in Examples 1 and 2.

[0078] HCl gas was added to the etching gas in an amount ranging from 0to 120 SCCM in this Example. Regarding the results observed aftercarrying out the etching under the conditions specified in Table 1, thechange in the etching rate of chromium when adding NH₃ to the etchinggas in an amount ranging from 0 to 120 SCCM and that in the etching rateof resist when adding NH3 in an amount ranging from 0 to 20 SCCM and therange/average are shown as a function of the added amount of NH₃ gas inFIG. 13. In FIG. 13, the results were obtained by using a chromium maskblank or a chromium mask blank on which the above-mentioned EB resist isapplied, without any patterning, as in this case of FIG. 7. As for theright longitudinal axis in FIG. 13, the range/average is the same asthat shown in FIG. 7.

[0079] Shown in FIG. 11 are the results observed when forming testpatterns (FIGS. 5(A) and (B)) which comprised dense and coarse patternsarranged in a plane. Namely, the change in the averaged dimensionaldifference between the coarse and dense portions (i.e., the differencein the dimension between the coarse and dense portions) as a function ofthe added amount of the NH₃ gas was shown in FIG. 11 while comparingthese results with those obtained in Example 1 in which hydrogen gas wasused as the added gas component. In addition, shown in the followingTable 2 are the change in the dimension of the coarse and dense portionsas a function of the amount of the NH₃ gas which is added to the etchinggas when the test patterns are formed through dry-etching, together withthe results obtained in Example 1 in which hydrogen gas is added andExample 2 in which HCl gas is added. Shown in the following Table 3 arethe dimensional variation in the coarse and dense portions of the resistpattern obtained after the development as a function of the added gasand an added amount thereof.

[0080] As will be seen from the results shown in Table 2 and FIG. 13,when adding the NH₃ gas to the etching gas, the etching rate of chromiumslightly increases until the added amount of the NH₃ gas reaches 60 SCCMand decreases as the added amount of the NH₃ gas increases even further.In addition, the etching rate of resist decreases a little until theadded amount of the NH₃ gas reaches 20 SCCM.

[0081] As will be seen from the data shown in Table 2 and FIG. 11, thereis observed a dimensional difference of about 0.075 μm between thecoarse and dense portions of the patterns when NH₃ gas was added in anamount 60 SCCM to the etching gas. Thus, it can be said that thisdimensional difference of about 0.075 μm corresponded to about {fraction(1/1.2)} times that (0.090 μm) observed when any NH₃ gas was not added.

[0082] Moreover, the etching operation may be safer by the use of NH₃gas in place of hydrogen gas as the added gas. It is difficult tofurther improve the dimensional difference of the coarse and denseportions of the patterns, even if the relative flow rate of the etchinggases were optimized. TABLE 1 Pattern-Forming Conditions Dry-EtchingConditions Resist Layer RF Mag- Inter- Just Total Added Thickness AddedPower Pres- netic electrode Etching Etching Ex. Etching Sample of ResistCl₂ O₂ Gas Supply sure Field Distance Time Time No. Gas No. Resist ÅSCCM SCCM SCCM W Pa Gauss mm sec sec 1 H₂ 1-1 ZEP810S 3000 (80) (20) (0)80 6.8 50-60 60 296 888 80 20 0 1-2 ″ ″ (72.73) (18.18) (9.09) ″ ″ ″ ″275 625 80 20 10 1-3 ″ ″ (65.79) (16.45) (17.76) ″ ″ ″ ″ 185 556 80 2021.6 1-4 ″ ″ (55.83) (13.96) (30.22) ″ ″ ″ ″ 205 615 80 20 43.3 1-5 ″ ″(46.48) (11.62) (41.89) ″ ″ ″ ″ 240 721 80 20 72.1 2 HCl 2-1 ZEP70004000 (80) (20) (0) 80 6.8 50-60 60 296 888 80 20 0 2-2 ″ ″ (69.57)(17.39) (13.04) ″ ″ ″ ″ 175 525 80 20 15 2-3 ″ ″ (61.54) (15.38) (23.08)″ ″ ″ ″ 174 522 80 20 30 2-4 ″ ″ (50.00) (12.50) (37.50) ″ ″ ″ ″ 195 58580 20 60 2-5 ″ ″ (36.36) (9.09) (54.54) ″ ″ ″ ″ 250 750 80 20 120 3 NH₃3-1 ZEP7000 4000 (100) (20) (0) 80 6.8 50-60 60 285 855 80 20 0 3-2 ″ ″(72.73) (18.18) (9.09) ″ ″ ″ ″ 290 870 80 20 10 3-3 ″ ″ (66.67) (16.67)(16.67) ″ ″ ″ ″ 283 849 80 20 20 3-4 ″ ″ (57.14) (14.29) (28.57) ″ ″ ″ ″251 753 80 20 40 3-5 ″ ″ (50.00) (13.50) (37.50) ″ ″ ″ ″ 244 732 80 2060

[0083] TABLE 2 Dimensions of Patterns Observed After Dry-EtchingDifference Between Coarse and Dense Dense Portion Coarse PortionPortions (Coarse-Dense) Flow Rate (Large Removed Area) (Small RemovedArea) [μm] Added of Added [μm] [μm] Average Ex. Etching Sample Gas L/SL/S ISO ISO L/S L/S ISO ISO L/S L/S ISO ISO (Absolute No. Gas No. SCCMLine Space Line Space Line Space Line Space Line Space Line Space Value)1 H₂ 1-1 0 1.974 2.029 1.970 2.021 2.067 1.942 2.065 1.938 0.093 −0.0870.095 −0.083 0.09 1-2 10 1.878 2.118 1.874 2.121 1.914 2.081 1.912 2.0740.036 −0.037 0.038 −0.047 0.040 1-3 21.6 1.969 2.030 1.967 2.032 1.9872.015 1.985 2.013 0.018 −0.015 0.018 −0.019 0.018 1-4 43.3 1.972 2.0251.972 2.027 1.981 2.020 1.984 2.017 0.009 −0.005 0.012 −0.010 0.009 1-572.1 1.967 2.032 1.958 2.035 1.976 2.023 1.969 2.027 0.009 −0.009 0.011−0.008 0.009 2 HCl 2-1 0 1.941 2.058 1.927 2.056 2.029 1.975 2.029 1.9760.088 −0.083 0.102 −0.080 0.088 2-2 15 1.897 2.093 1.896 2.084 1.9382.063 1.936 2.059 0.041 −0.030 0.040 −0.025 0.035 2-3 30 1.895 2.0991.892 2.086 1.931 2.069 1.921 2.067 0.036 −0.030 0.029 −0.019 0.029 2-460 1.905 2.091 1.893 2.098 1.930 2.065 1.923 2.070 0.025 −0.026 0.030−0.028 0.027 2-5 120 1.889 2.108 1.881 2.110 1.937 2.056 1.930 2.0580.048 −0.052 0.049 −0.052 0.050 3 NH₃ 3-1 0 1.947 2.053 1.948 2.0572.043 1.967 2.038 1.968 0.096 −0.086 0.090 −0.089 0.090 3-2 10 1.9382.060 1.934 2.061 2.034 1.975 2.030 1.971 0.096 −0.085 0.096 −0.0900.092 3-3 20 1.941 2.050 1.945 2.045 2.039 1.964 2.038 1.969 0.098−0.086 0.093 −0.076 0.088 3-4 40 1.916 2.077 1.912 2.075 2.008 1.9972.003 1.995 0.092 −0.080 0.091 −0.080 0.086 3-5 60 1.908 2.088 1.9022.089 1.988 2.014 1.978 2.018 0.080 −0.074 0.076 −0.071 0.075

[0084] TABLE 3 Dimensions of Resist Patterns Observed After DevelopmentDifference Between Coarse and Dense Dense Portion Coarse PortionPortions(Coarse-Dense) Flow Rate (Large Removed Area) (Small RemovedArea) [μm] Added of Added [μm] [μm] Average Ex. Etching Sample Gas L/SL/S ISO ISO L/S L/S ISO ISO L/S L/S ISO ISO (Absolute No. Gas No. SCCMLine Space Line Space Line Space Line Space Line Space Line Space Value)1 H₂ 1-1 0 2.079 1.440 2.081 1.439 2.116 1.391 2.101 1.400 0.037 −0.0490.021 −0.039 — 1-2 10 2.090 1.428 2.092 1.424 2.112 1.396 2.112 1.4000.022 −0.032 0.020 −0.024 — 1-3 21.6 2.091 1.420 2.067 1.425 2.113 1.3952.107 1.387 0.022 −0.025 0.040 −0.038 — 1-4 43.3 2.092 1.422 2.099 1.4122.132 1.373 2.129 1.375 0.040 −0.049 0.030 −0.037 — 1-5 72.1 2.070 1.4152.082 1.420 2.128 1.382 2.125 1.386 0.058 −0.033 0.043 −0.034 — 2 HCl2-1 0 2.076 1.394 2.066 1.394 2.106 1.379 2.117 1.362 0.030 −0.015 0.051−0.032 — 2-2 15 2.072 1.404 2.063 1.391 2.113 1.358 2.113 1.352 0.041−0.046 0.050 −0.039 — 2-3 30 2.064 1.391 2.063 1.402 2.103 1.352 2.1131.351 0.039 −0.039 0.050 −0.051 — 2-4 60 2.064 1.395 2.062 1.394 2.1081.357 2.100 1.355 0.044 −0.038 0.038 −0.039 — 2-5 120 2.063 1.383 2.0531.393 2.107 1.362 2.102 1.353 0.044 −0.021 0.049 −0.040 — 1 NH₃ 3-1 02.082 1.384 2.080 1.371 2.130 1.332 2.129 1.336 0.048 −0.052 0.049−0.035 — 3-2 10 2.073 1.392 2.072 1.387 2.116 1.339 2.116 1.341 0.043−0.053 0.044 −0.046 — 3-3 20 2.071 1.386 2.070 1.379 2.110 1.358 2.1131.356 0.039 −0.028 0.043 −0.023 — 3-4 40 2.074 1.383 2.062 1.383 2.1111.350 2.100 1.354 0.037 −0.033 0.038 −0.029 — 3-5 60 2.067 1.399 2.0531.389 2.101 1.361 2.093 1.343 0.034 −0.038 0.040 −0.046 —

EXAMPLE 4

[0085] In the light of the results obtained in Examples 1 and 2, it wasfound that the use of hydrogen and hydrogen chloride gases indry-etching as the gas component to be added to the etching gas permitsthe reduction of the dimensional difference due to the coexistence ofcoarsely and densely patterned portions in a plane and the formation ofhigh precision patterned products by etching. Accordingly, there wereprepared several kinds of photomasks according to the procedures used,in Examples 1 and 2, for preparing the test samples carrying testpatterns, or by forming a resist layer on a Cr photomask blank, followedby a series of well-known pattern-forming steps such as light-exposure,development, etching and washing. The resulting photomasks each hadpatterns such as hole systems or line-and-space patterns to betransferred to a wafer and comprised, on a plane, coexisting coarse anddense patterns. The resulting photomask was found to have a very smalldimensional difference between the coarse and dense portions of thepatterns. These results clearly indicate that the use of the foregoingadded gas permits the achievement of quite satisfactory results, i.e.,substantial reduction of the dimensional difference, like the resultsobserved in Examples 1 and 2.

EXAMPLE 5

[0086] A semiconductor circuit was formed on a wafer by repeating thefollowing steps (1) to (4) using the photomask prepared in Example 4:

[0087] (1) A light-sensitive material was applied onto the wafer;

[0088] (2) The patterns on the photomask were scaled down andtransferred to the wafer using a stepper (scale factor: ⅕, ¼ or ½);

[0089] (3) The wafer provided thereon with the exposed light-sensitivematerial was developed to form a resist pattern on the wafer;

[0090] (4) The wafer was subjected to dry-etching or ion-implantationthrough the resist pattern.

[0091] The semiconductor circuits thus produced were a memory circuit(FIG. 14) whose patterns were regularly arranged, a logic circuit whosepatterns were randomly arranged and a system LSI circuit (see FIG. 15)comprising combined memory and logic circuits.

[0092] The memory circuit as shown in FIG. 14 comprises a memory circuitportion whose patterns are regularly arranged and a peripheral circuitportion in which patterns are irregularly arranged in order to ensurethe connection to the exterior and these circuit portions differ fromeach other in the rate of the area occupied by the patterns. In thetransistor gate-forming process which has a serious influence on thecharacteristics of the circuit, the rate of the area removed forpatterning on the peripheral circuit portion increases in proportion toan increase in the size of the memory cell portion, i.e. is higher thanthat of the memory cell portion. In the case of the semiconductorcircuit produced according to the present invention, the dimensionaldifference observed between the memory cell and peripheral circuitportions is very small and on the order of about 0.004 μm and thisindicates that the amount of the dimensional variation is not more than2%. Consequently, the patterns never adversely affect the characteristicproperties of the resulting circuit. Thus, satisfactory semiconductorcircuits could be produced according to the present invention, which didnot show any difference between the memory cell and peripheral circuitportions in characteristic properties.

[0093] Moreover, the semiconductor circuit or logic circuit produced inthis Example was found to have excellent quality, since the same effectdescribed above could also be attained by the logic circuit in whichpatterns were randomly distributed and the areas removed for patterningwere also randomly distributed and the dimensional difference within thechip was found to be very small on the order of not more than 0.004 μm.It was also found that the present invention permitted the production ofan excellent semiconductor circuit or a system LSI circuit comprisingcombined memory and logic circuit portions (FIG. 15), which did notdiffer from each other in characteristic properties. This is because thememory and logic circuit portions differ from each other in the densityof patterns, but the dimensional difference between the memory cell andlogic circuit portions is very small on the order of 0.004 μm andtherefore, the patterns never adversely affect the characteristicproperties of the resulting circuit.

[0094] As has been discussed above in detail, the method of the presentinvention permits the decrease of the dimensional difference due to thecoexistence of coarsely and densely patterned portions in a plane andthe production of a high precision pattern-etched product by using amixed gas, which comprises (a) a reactive ion etching gas as a mixtureof an oxygen-containing gas and a halogen-containing gas, and (b) areducing gas containing at least hydrogen, in the dry-etching process asa means for forming fine patterns.

[0095] Moreover, the method for preparing a photomask according to thepresent invention permits a photomask which has a small dimensionaldifference due to the coexistence of the coarse and dense patterns in aplane and whose patterns are highly precisely processed. In addition,the photomask has uniform patterns. Further the semiconductor circuitproduced using the photomask of the present invention has a very highintegration density.

[0096] In addition, the use of hydrogen chloride gas instead of hydrogengas as the added gas component can ensure safer etching procedures.

What is claimed is:
 1. A dry-etching method which comprises dry-etchinga metal thin film, wherein the method is characterized by using, as anetching gas, a mixed gas including (a) a reactive ion etching gas, whichcontains an oxygen-containing gas and a halogen-containing gas, and (b)a reducing gas added to the gas component (a), in the process fordry-etching the metal thin film.
 2. The dry-etching method as set forthin claim 1 wherein said metal thin film is a thin film of a memberselected from the group consisting of Al, Au, Pt, Ag, Si, Ge, Cr, Fe,Cu, Ni, Ta, Mo, W, Zr and an alloy of two or more of these metals. 3.The dry-etching method as set forth in claim 1 wherein said metal thinfilm is a thin film selected from the group consisting of a metal film,a metal oxide film, a metal nitride film, a metal fluoride film and alaminated film thereof.
 4. The dry-etching method as set forth in claim2 wherein said metal thin film is a thin film selected from the groupconsisting of a metal film, a metal oxide film, a metal nitride film, ametal fluoride film and a laminated film thereof.
 5. The dry-etchingmethod as set forth in claim 1 wherein said reducing gas is a gascontaining at least hydrogen.
 6. The dry-etching method as set forth inclaim 2 wherein said reducing gas is a gas containing at least hydrogen.7. The dry-etching method as set forth in claim 1 wherein said reducinggas is hydrogen gas; a hydrocarbon gas selected from the groupconsisting of C_(n)H_(2n+2) (n=1 to 8), C_(n)H_(2n) (n=2 to 10),C_(n)H_(2n−2) (n=2 to 8); an alcoholic gas selected from the groupconsisting of CH₃OH, C₂H₅OH, CH₃CH₂CH₂OH, (CH₃)₂CHOH, (CH₃)₃COH,CH₂=CHCH₂OH; a hydrogen halide gas selected from the group consisting ofHF, HCl, HBr and HI; ammonia gas; or water.
 8. The dry-etching method asset forth in claim 2 wherein said reducing gas is hydrogen gas; ahydrocarbon gas selected from the group consisting of C_(n)H_(2n+2) (n=1to 8), C_(n)H_(2n) (n=2 to 10), C_(n)H_(2n−2) (n=2 to 8); an alcoholicgas selected from the group consisting of CH₃OH, C₂H₅OH, CH₃CH₂CH₂OH,(CH₃)₂CHOH, (CH₃)₃COH, CH₂=CHCH₂OH; a hydrogen halide gas selected fromthe group consisting of HF, HCl, HBr and HI; ammonia gas; or water. 9.The dry-etching method as set forth in claim 1 wherein said metal thinfilm is a chromium-containing film, said mixed gas comprises chlorinegas, oxygen gas and hydrogen gas and the relative flow rates of thesegases are 73 to 46, 19 to 11 and 9 to 42% by volume, respectively. 10.The dry-etching method as set forth in claim 2 wherein said metal thinfilm is a chromium-containing film, said mixed gas comprises chlorinegas, oxygen gas and hydrogen gas and the relative flow rates of thesegases are 73 to 46, 19 to 11 and 9 to 42% by volume, respectively. 11.The dry-etching method as set forth in claim 1 wherein said metal thinfilm is a chromium-containing film, said mixed gas comprises chlorinegas, oxygen gas and hydrogen chloride gas and the relative flow rates ofthese gases are 70 to 36, 18 to 9 and 13 to 55% by volume, respectively.12. The dry-etching method as set forth in claim 2 wherein said metalthin film is a chromium-containing film, said mixed gas compriseschlorine gas, oxygen gas and hydrogen chloride gas and the relative flowrates of these gases are 70 to 36, 18 to 9 and 13 to 55% by volume,respectively.
 13. A method for preparing a photomask by performing aseries of pattern-forming steps such as a step for forming a resistlayer on a photomask blank, a step for exposing and patterning saidresist layer, a developing step, a step for etching said photomask blankand a step for removing the resist layer, wherein the method ischaracterized in that patterns to be transferred onto a wafer are formedon said photomask blank according to the dry-etching method as set forthin claim 1 to thus give a photomask.
 14. A method for preparing aphotomask by performing a series of pattern-forming steps such as a stepfor forming a resist layer on a photomask blank, a step for exposing andpatterning said resist layer, a developing step, a step for etching saidphotomask blank and a step for removing the resist layer, wherein themethod is characterized in that patterns to be transferred onto a waferare formed on said photomask blank according to the dry-etching methodas set forth in claim 2 to thus give a photomask.
 15. A photomask whichis prepared by performing a series of pattern-forming steps such as astep for forming a resist layer on a photomask blank, a step forexposing and patterning said resist layer, a developing step, a step foretching said photomask blank and a step for removing said resist layer,wherein the photomask is characterized in that patterns to betransferred onto a wafer are formed on said photomask blank according tothe dry-etching method as set forth in claim
 1. 16. A photomask which isprepared by performing a series of pattern-forming steps such as a stepfor forming a resist layer on a photomask blank, a step for exposing andpatterning said resist layer, a developing step, a step for etching saidphotomask blank and a step for removing said resist layer, wherein thephotomask is characterized in that patterns to be transferred onto awafer are formed on said photomask blank according to the dry-etchingmethod as set forth in claim
 2. 17. A method for manufacturing asemiconductor circuit which comprises the steps of transferring thepatterns formed on the photomask as set forth in claim 15 on a wafer onwhich a light-sensitive material is coated, developing saidlight-sensitive material to form resist patterns on the wafer, tomanufacture a semiconductor circuit which comprises coexisting coarseand dense patterns corresponding to said resist patterns.
 18. Asemiconductor circuit having a circuit which comprises coexisting coarseand dense patterns corresponding to the resist patterns formed bytransferring said resist patterns formed on the photomask as set forthin claim 15 on a wafer on which a light-sensitive material is coated andthen developing said light-sensitive material.
 19. A dry-etchingapparatus used in dry-etching a metal thin film, wherein the apparatusis provided with a sequencer for establishing dry-etching conditions,wherein said metal thin film to be dry-etched is a chromium-containingfilm, wherein if an etching gas used consists of chlorine, oxygen andhydrogen gases, the relative flow rates of these gases as expressed interms of % by volume range from 73 to 46, 19 to 11 and 9 to 42% byvolume, respectively, or if an etching gas used consists of chlorine,oxygen and hydrogen chloride gases, the relative flow rates of thesegases as expressed in terms of % by volume range from 70 to 36, 18 to 9and 13 to 55% by volume, respectively, and wherein the apparatus is sodesigned that when inputting the parameters relating to the foregoingdry-etching conditions, directly or through a memory device of acomputer, to said sequencer and then starting the dry-etching process,the dry-etching is automatically carried out under the foregoingdry-etching conditions.
 20. A dry-etching apparatus comprising anetching chamber, a transport chamber, a substrate cassette bed and asequencer for establishing dry-etching conditions, wherein fourelectromagnets each comprising a square-shaped ring-like coil areprovided on an outer side of said etching chamber, two each of theseelectromagnets being opposite to one another and making a pair, theseelectromagnets being so designed that when applying a low frequencycurrent which is 90 deg. out of phase thereto, the combined magneticfield established by these two paired electromagnets can rotate in aplane parallel to a substrate at a frequency identical to that of thelow frequency current, an RF electrode and an opposite electrode aredisposed in said etching chamber, a transport robot for transportingsaid substrate is provided in said transport chamber, said transportrobot being a two-joint robot having two knots, the tip of a transportarm thereof being able to undergo advancing, reciprocating and rotatingmotions due to the composition of rotational motions of a motor axis andthese two knots within each horizontal plane, the robot thustransporting the substrate, wherein a metal thin film to be dry-etchedis a chromium-containing film, wherein if an etching gas used consistsof chlorine, oxygen and hydrogen gases, the relative flow rates of thesegases as expressed in terms of % by volume range from 73 to 46, 19 to 11and 9 to 42% by volume, respectively, or if an etching gas used consistsof chlorine, oxygen and hydrogen chloride gases, the relative flow ratesof these gases as expressed in terms of % by volume range from 70 to 36,18 to 9 and 13 to 55% by volume, respectively, and wherein the apparatusis so designed that when inputting the parameters relating to theforegoing dry-etching conditions, directly or through a memory device ofa computer, to said sequencer and then starting the dry-etching process,the dry-etching is automatically carried out under the foregoingdry-etching conditions.